Amphetamine is a perfect example of how the Mac isn't a walled garden. They always had the option top sell outside the App Store. That is fundamentally the difference between what makes a platform a walled garden. They might have lost some sales because they couldn't participate in the Mac App Store, but they could still sell their product. Some companies choose to avoid the Mac App Store because they don't like Apple's policies.
That's neither here nor there.
(a) Amphetamine could still be sold outside the Mac App Store.
(b) An app name could be problematic even in FOSS land. It's just that instead of Amphetamine being the name that causes it, it will be something else. E.g. with the trend of banning/changing terms like "master" (as in replication primary master, not as in the owner of slaves), unfortunately named apps could be thrown out something or ask to be renamed to be included in a distros package manager or a project.
Ask Amphetamine about how open they are.